In a prior art fluid machine, as disclosed, for example, in Japanese Patent No. 2540738, a compressor device for a vapor compression refrigerating system is commonly used as an expansion device, and the compressor device is used as the expansion device when energy is collected by Rankine cycle.
In the above fluid machine, in which the expansion device and the compressor device are commonly used, the capacity for the fluid machine is generally set to such an amount necessary for the compressor device on the condition that the refrigerating cycle is efficiently operated. As a result, the capacity for the fluid machine as the expansion device is inevitably decided by such amount. Accordingly, the design flexibility for the expansion device is reduced and an optimum efficiency of the fluid machine for collecting the waste heat can be hardly obtained.
For example, the pressure of the refrigerant in the operation of the Rankine cycle is higher than that in the operation of the refrigerating cycle, a volumetric flow rate for the operation of Rankine cycle becomes smaller than that for the refrigerating cycle, even if the amount (weight) of the refrigerant is the same. Then, a rotational speed of the fluid machine becomes lower when it is operated as the expansion device, an influence of refrigerant leak per revolution would become larger (leak speed becomes higher than expansion speed), and thereby the efficiency of the expansion device is decreased.